1. Field of the Invention
This invention relates to a digital video signal processing device, and more particularly to an improvement of a rate converter for converting a sampling rate of digital video signal.
2. Description of the Prior Art
In conventional rate converters for digital video signals, there are some which are capable of converting especially the component digital video signal of D-1 525/60 standard in the SMPTE (hereinafter referred to as a D-1 digital video signal) into the D-2 NTSC standard composite digital video signal (hereinafter referred to as a D-2 digital video signal).
Here, since the sampling frequency of D-1 digital video signal consists of 13.5 [MHz] while the sampling frequency of D-2 digital video signal is 14.3 [MHz] and they have the correlation of 33:35, there are many cases where the high over-sampling filters are constituted in the rate converters in utilizing the integer ratio.
In this case, it is a common practice that the filters to constitute in order to coincide the sample points each other periodically, and accordingly, regarding the signal element of effective part of video signal, the rate can be converted correctly.
Then, practically, according to the formats of digital video signal of D-1 and digital video signal of D-2, whereas the digital video signal of D-1 has a sample point, D-2 has no sample point in regard to 50, percent point of the dropping of sync signal (hereinafter referred to the sync point).
Accordingly, as discussed above, in the case where the filter is constituted in order that two sample points are coincident each other at an optional cycle, there is a problem that a wave form of the sync signal of D-1 digital video signal cannot be converted correctly to conform to the format of D-2 digital video signal.
Heretofore, as shown in FIG. 1, there is a rate converter 1 which generates and adds the sync signal or burst signal to conform to the format of D-2 digital video signal aside from the rate converter of effective part in the D-1 digital video signal.
In this rate converter 1, the digital video signal VD1 of D-1 is decoded via the 4:2:2 decoder (not shown) and the resultant luminance signal Y1 is inputted to the luminance rate converter 2 which is composed of the over-sampling filter, and the sampling rate is converted and the resultant luminance signal Y2 after being converted will be inputted to the first composing circuit 3.
And similarly, the color difference signals R-Y, B-Y to be obtained by decoding the digital video signal VD1 of D-1 will be inputted to the chrominance rate converters 4 and 5 which comprise the over-sampling filter respectively and the resultant post converted color difference signal will be modulated at a modulator 6 and the post modulated color signal C2 will be inputted to a composing circuit 3.
In this first composing circuit 3, the digital video signal VD2 of D-2 is generated by composing the luminance signal Y2 and the color signal C2 to be inputted, and then outputted to the second composing circuit 7. The sync signal SYNC and the burst signal BST corresponding to the D-2 format outputted from the sync additional circuit 8 are inputted to the second composing circuit 7.
With this arrangement, in the second composing circuit 7 as shown in FIG. 2A, a digital video signal VD20 of D-2 is obtained after combining the sync signal SYNC and the burst signal BST corresponding to the D-2 format in the effective part of the video signal of the digital video signal VD2 and will be outputted as an output of the rate converter 1.
However, in case of converting the rate using this kind of rate converter 1, as shown in FIG. 2A by a dashed line, there occurs a slight phase difference between the residual chroma and the regular chroma on the luminance signal because of the time base error, and accordingly the distance between the sync signal SYNC and the effective part in the video signal, i.e., the video phase may be distorted.
As against the dropping of sync signal SYNC is converted with the same correct phase as the sync signal SYNC of the digital video signal of D-1 as shown in FIG. 2B by a line, the phase difference occurs in the case where the video phase error deviates for .DELTA.V and gap grows bigger by repeating the rate conversions and shown in FIG. 2B by a dashed line, and it has been found insufficient as a solution for practical use.